1. Field of the Invention
The present invention relates to a system and method for recovering spent or waste sulfuric acid liquid and reprocessing it, in particular a system and method for continuously reprocessing waste sulfuric acid liquid wherein sulfuric acid liquid which has enough concentration and impurity for reuse can be provided in a stable and continuous way. The present invention also relates to a heater supporting structure for a heating device made of glass, which is used as a heating device for corrosive liquid such as sulfuric acid and nitric acid.
2. Discussion of Background
In a semiconductor fabrication process, sulfuric acid liquid has been used for cleaning of wafers and removal of photo resists.
The waste sulfuric acid which has been used in such process has been disposed after neutralized, taken over to dealers for disposal of industrial waste, or concentrated and distilled for recycling.
As such systems for reprocessing waste sulfuric acid liquid have been known a continuous system including a concentration step and a distillation step which is disclosed in U.S. Pat. No. 3,294,650, and a batch processing system for production of reprocessed sulfuric acid liquid for batteries which is disclosed-in Intreprinderea Chemida xe2x80x9cDudestixe2x80x9d Rom. RO 83,474, May 30, 1984, application Ser. No. 103,545, Feb. 27, 1981; 3pp CA 1985, 103(4) 56183z.
As conventional systems for reprocessing waste sulfuric acid liquid have been known a normal pressure type apparatus which is operated at substantially atmospheric pressure, and a reduced pressure type apparatus which is provided with a vacuum device such as a vacuum pump and is operated under reduced pressure from a few Torr (1xc3x97103 Pa) to tens of Torr (5xc3x97103 Pa). Although a continuous system is adopted in such a normal pressure type apparatus because of easy removal of each liquid, the material of a gasket usable to pipe connecting portions for each liquid is limited because the operating temperature is 330xc2x0 C. or higher. If the gasket is made of e.g. metal or carbon, there is a possibility that impurities might mix. If the operation is carried out without use of the gasket for that reason, there has been created a problem in that the apparatus and pipes could be broken by occurrence of leakage of each liquid at the connecting portions and difficulty in absorption of expansion and contraction of the system and the pipes due to a temperature difference.
In the conventional apparatuses, heating vessels are of a vertical type or a horizontal type without a baffle plate. In the concentration step, it has been unstable to concentrate the liquid to a target level. In order to cope with this problem in such a reduced pressure type apparatus, there have been adapted a semi-batch system wherein the process flow is cut off so that concentrated sulfuric acid liquid from a concentrating vessel is held in an intermediate tank and is transferred into the distillation step after determining the concentration of the concentrated sulfuric acid liquid in the intermediate tank, or a system wherein switching to the distillation operation is made after finding the concentration state by estimating the concentration state based on an evaporating temperature in a batch system or by measuring the concentration of the concentrated sulfuric acid liquid which is taken out of the concentrating vessel through a branch pipe and is cooled. Such an normal pressure type apparatus has created a problem in that the apparatus and the pipes are quite likely to be broken because the evaporating temperature is remarkably raised to a specified value or higher to maintain high concentration. The normal pressure apparatus is insufficient in terms of stable and continuous operation.
As the heating vessel for concentrating or distilling corrosive liquid such as sulfuric acid or nitric acid is usually used a heating vessel made of the in terms of corrosion resistance.
As the heating source for such a heating vessel have been used hot water, steam, heat medium and electricity. The heating vessel has been heated by such a heating source from within, from outside or from within and outside. The heating from within has usually been adopted since the heating from outside creates a problem in that there is much heat loss due to radiation of heat.
When the heating is carried out from within, there has been adopted a supporting structure wherein a heater to be provided in the heating vessel is supported only by a heater inserting portion of the heating vessel in a cantilevered way.
Since there is a possibility that such a supporting structure could break the heater or the heating vessel made of glass, it is required that the heater have a length less than 1000 mm or the inside of the heating vessel be slowly heated so as to prevent the liquid in the vessel for vibrating. If there is no other way but to heat the inside of the vessel intensely, the heater and the vessel have been used being ready for breakage, which has required that the heater and the vessel are repaired or exchanged with new ones periodically.
Although it is proposed that a plurality of short heaters are arranged to cope with this problem, this proposal requires that the heating vessel must become larger, creating a problem in terms of apparatus cost.
It is an object of the present invention to solve the problems of the prior art.
The foregoing and other objects of the present invention have been attained by providing a system for continuously reprocessing waste sulfuric acid liquid, comprising:
an apparatus for concentration including
a heating vessel for concentration having a supply device for continuously supplying waste sulfuric acid liquid thereto, an inlet for receiving the waste sulfuric acid liquid, and a heating device, and
a condenser for cooling and condensing vaporized sulfuric acid led out of the heating vessel;
an apparatus for distillation including
a heating vessel for distillation having an inlet for continuously receiving concentrated sulfuric acid liquid, and a heating device, and
a condenser for distillation for cooling and condensing vaporized sulfuric acid led out of the heating vessel for distillation; and
a connecting pipe for continuously removing the concentrated sulfuric acid liquid through a bottom of the heating vessel for concentration and supplying the concentrated sulfuric acid liquid to the heating vessel for distillation;
wherein the heating vessel for concentration is elongated in a horizontal direction, and the heating vessel for concentration has a baffle plate provided therein so that the sulfuric acid liquid which is supplied through the inlet of the heating vessel for concentration is gradually concentrated so as to have higher concentration at an outlet of the heating vessel for concentration than concentration at the inlet of the heating vessel for concentration.
The present invention also provides a method for continuously reprocessing waste sulfuric acid liquid, comprising:
concentrating waste sulfuric acid liquid by an apparatus for concentration including
a heating vessel for concentration having a supply device for continuously supplying waste sulfuric acid liquid thereto, an inlet for receiving the waste sulfuric acid liquid, and a heating device, and
a condenser for cooling and condensing vaporized sulfuric acid let out of the heating vessel; and
distilling the concentrated waste sulfuric acid liquid by an apparatus for distillation including
a heating vessel for distillation having an inlet for continuously receiving the concentrated sulfuric acid liquid, and a heating device, and
a condenser for distillation for cooling and condensing vaporized sulfuric acid let out of the heating vessel for distillation;
wherein the heating vessel for concentration is elongated in a horizontal direction, and the heating vessel for concentration has a baffle plate provided therein so that the sulfuric acid liquid which is supplied through the inlet of the heating vessel for concentration is gradually concentrated so as to have higher concentration at an outlet of the heating vessel for concentration than concentration at the inlet of the heating vessel for concentration.
In order to control the operation of the heating vessel for concentration, the heating vessel for concentration may be provided with a liquid level measuring device having a liquid level detector for sulfuric acid liquid, and may have the outlet for the concentrated sulfuric acid liquid provided with a temperature detector. As such a liquid level detector may be used a capacitance-operated liquid level detector. The heating vessel for concentration may have an outer surface formed with a liquid level gauge pipe, which is incorporated into an optical liquid level sensor using infrared rays as the liquid level sensor.
It is preferable that the heating vessel for concentration has a packing material layer arranged at an upper portion therein, and a partial condenser for vaporized sulfuric acid is arranged above the packing material layer.
In order to remove the condensed liquid in the apparatus for concentration, there may be provided a condensed liquid receiving tank having a liquid inlet, a liquid outlet and a gas outlet, and a condensed removing tank having a liquid inlet, a liquid outlet, a gas inlet and a gas outlet and having valves in the inlets and the outlets. In the removal of the condensed liquid, gas which is led into the condensed removing tank and has pressure not less than normal pressure may be nitrogen gas which has been cleaned by a filter. A detector which detects that a predetermined volume of condensed liquid has been stored in the condensed liquid receiving tank may be an optical liquid level sensor using infrared rays. A time control may be adopted to repeat the removal at preset time intervals.
In the system for continuously reprocessing waste sulfuric acid liquid according to the present invention, at least one baffle plate which has a liquid passing hole formed therein may be provided between the inlet for the waste sulfuric acid liquid and the outlet for the concentrated sulfuric acid liquid in the heating vessel for concentration. A plurality of baffle plates may be provided in the heating vessel for concentration. A baffle plate may have a height shorter than that of the heating vessel for concentration, and the liquid level of the waste sulfuric acid liquid may be controlled to be slightly higher than the baffle plate, which allows the liquid to pass without provision of the liquid passing hole in the baffle plate.
In order to control the operation of the heating vessel for distillation, the heating vessel for distillation may be provided with a liquid level measuring device having a liquid level detector for the sulfuric acid liquid, and may have the outlet for the waste sulfuric acid liquid provided with a temperature detector. As the liquid level detector may be used a capacitance-operated liquid level detector. The heating vessel for distillation may have an outer surface formed with a liquid level gauge pipe, which is incorporated into an optical liquid level sensor using infrared rays as the liquid level sensor.
The heating vessel for distillation may have a packing material layer arranged at an upper portion therein, and a return pipe for condensed liquid of sulfuric acid may be provided above the packing layer material in the heating vessel for distillation to return the condensed liquid of sulfuric acid from the condenser for distillation. The return pipe for condensed liquid of sulfuric acid may have a valve or an orifice provided therein.
In order to remove the condensed liquid of sulfuric acid in the apparatus for distillation, there may be provided a condensed liquid receiving tank for sulfuric acid having a liquid inlet, a liquid outlet and a gas outlet, and a condensed liquid removing tank for sulfuric acid having a liquid inlet, a liquid outlet, a gas inlet and a gas outlet and having valves arranged in the inlets and the outlets. In the removal of the condensed liquid of sulfuric acid, gas which is led into the condensed liquid receiving tank and has pressure not less than normal pressure may be nitrogen gas which has been cleaned by a filter. A detector which detects that a predetermined volume of condensed liquid of sulfuric acid has been stored in the condensed liquid receiving tank may be an optical liquid level sensor using infrared rays. A time control may be adopted to repeat the removal at preset time intervals.
In the system for continuously reprocessing waste sulfuric acid liquid, a baffle plate which has a liquid passing hole formed therein may be arranged between the inlet for the concentrated sulfuric acid liquid and the outlet of the waste sulfuric acid liquid in the heating vessel for distillation. A plurality of baffle plates may be arranged in the heating vessel for distillation. A baffle plate may have a height shorter than that of the heating vessel for distillation, and the liquid level of the concentrated sulfuric acid liquid may be controlled to be slightly higher than the baffle plate, which allows the liquid to pass without provision of the liquid passing hole in the baffle plate.
In order to remove the waste sulfuric acid liquid in the apparatus for distillation, there may be provided a waste sulfuric acid receiving tank having a liquid inlet, a liquid outlet and a gas outlet, and a waste sulfuric acid removing tank having a liquid inlet, a liquid outlet, a gas inlet and a gas outlet and having valves arranged in the inlets and the outlets. In the removal of the waste sulfuric acid liquid, gas which is led into the waste sulfuric acid receiving tank and has pressure less than normal pressure may be nitrogen gas which has been cleaned by a filter. A detector which detects that a predetermined volume of sulfuric acid liquid has been stored in the waste sulfuric acid receiving tank may be an optical liquid level sensor using infrared rays. A time control may be adopted to repeat the removal at preset time intervals.
The system for continuously reprocessing waste sulfuric acid liquid may include an evacuation device which depressurizes the inside of the apparatus for concentration, the apparatus for distillation and-the connecting pipe. Connecting portions of the connecting pipe with the apparatus for concentration and the apparatus for distillation may use a gasket made of fluorocarbon resin. In the system for continuously reprocessing waste sulfuric acid liquid, at least the condenser for distillation, the condensed liquid receiving tank for sulfuric acid, the condensed liquid removing tank for sulfuric acid and pipes for connecting these members in the apparatus for distillation may be made of silica glass.
In a preferred mode of the present invention, in the control of the heating vessel for concentration, the supply device for supplying waste sulfuric acid liquid to the heating vessel for concentration is controlled to regulate the supply of the waste sulfuric acid liquid based on the liquid level measured by the liquid level measuring device with the sulfuric acid liquid level detector, and the heating device of the heating vessel for concentration is controlled to regulate the temperature of the waste sulfuric acid liquid therein based on the temperature of the concentrated sulfuric acid liquid measured by the temperature measuring device with the temperature detector at the outlet for the concentrated sulfuric acid liquid in the heating vessel for concentration. The liquid level of the waste sulfuric acid liquid in the heating vessel for concentration can be regulated to be in a predetermined range of height so as to prevent a drop in the liquid level from causing the heating of the vessel in an empty state, to avoid insufficient evaporation and apparatus breakage due to a raise in the liquid level, and to adjust the flow of the liquid passing through the hole in the baffle plate. The concentrated sulfuric acid liquid can be heated to the boiling point for a predetermined concentration under temperature control. In that manner, the sulfuric acid liquid can be concentrated in a stable and continuous manner.
In a preferred mode of the present invention, the packing material layer which is provided in the apparatus for concentration works to catch sprays of waste sulfuric acid caused in generation of vaporized sulfuric acid, preventing the sprays entrained on the vaporized sulfuric acid from reaching the condenser. The partial condenser which is provided in the apparatus for concentration condenses part of the vaporized sulfuric acid, and work to return the condensed liquid of sulfuric acid to the heating vessel for concentration after the condensed liquid has wetted the packed layer, assisting the packed layer in catching the sprays. In that manner, the volume of the sulfuric acid included in the vapor which enters the vacuum device can be reduced to concentrate the waste sulfuric acid liquid in a stable and continuous way.
In a preferred mode of the present invention, the valves in the condensed liquid removing tank are controlled to be opened and closed in a preset order and at preset time intervals, removing the condensed liquid from a depressurized state into a normal pressure state. This allows the condensed liquid to be continuously received in the condensed liquid receiving tank without stopping the operation of the apparatus for concentration.
In a preferred mode of the present invention, if a plurality of baffle plates are arranged in the heating vessel for concentration, the function of the baffle plates is as follows: First, the waste sulfuric acid liquid which has been introduced into the heating vessel is stayed by a first baffle plate near to the inlet. Next, the waste sulfuric acid liquid is introduced into the space between the first baffle plate and a second baffle plate through the liquid passing hole in the first baffle plate, and the waste sulfuric acid liquid is stayed in the space between the first and second baffle plates. After such processes are repeated by the number of the baffle plates, the waste sulfuric acid liquid reaches the outlet for the concentrated sulfuric acid liquid in the heating vessel for concentration. Although the sulfuric acid concentration of the waste sulfuric acid liquid in the heating vessel for concentration is raised by evaporation due to heating, the reverse flow mixture of the waste sulfuric acid liquid through the liquid passing hole in the baffle plates is minimized in comparison with the absence of the baffle plates. The sulfuric acid concentration in the space between the first and second baffle plates becomes slightly higher than that at the inlet. After such an increase in the concentration is repeated by the number of the baffle plates, the waste sulfuric acid liquid reaches the outlet for the concentrated sulfuric acid liquid in the heating vessel for concentration, through which the concentrated sulfuric acid liquid having the concentration raised to a predetermined level is removed in a stable and continuous way. In that manner, the concentrated sulfuric acid liquid can be continuously supplied to the apparatus for distillation.
In a preferred mode of the present invention, in the control of the heating vessel for distillation, the waste sulfuric acid liquid supply device is controlled to regulate the supply of the waste sulfuric acid liquid based on the liquid level measured by the liquid level measuring device with the sulfuric acid liquid level detector, and the heating device for the heating vessel for distillation is controlled to regulate the evaporation quantity of the concentrated sulfuric acid liquid in the heating vessel for distillation based on the temperature of the concentrated sulfuric acid liquid measured by the temperature measuring device with the temperature detector in the outlet for the waste sulfuric acid liquid of the heating vessel. The liquid level of the concentrated sulfuric acid liquid in the heating vessel can be regulated to be in a predetermined range of height, preventing a drop in the liquid level from causing of heating of the heating vessel in an empty state, avoiding insufficient evaporation and apparatus breakage due to a raise in the liquid level, and adjusting the flow of the liquid passing through the hole in the baffle plate. In that way, the sulfuric acid liquid can be stably and continuously distilled.
In a preferred mode of the present invention, the packing material layer which is provided in the apparatus for distillation works to catch sprays of the concentrated sulfuric acid liquid caused in generation of vapor, preventing the sprays entrained on the vapor from reaching the condenser for distillation. The condensed sulfuric acid return pipe which is provided in the apparatus for distillation works to return the condensed sulfuric acid to the heating vessel for distillation after the condensed sulfuric acid has wetted the packed layer, assisting the packed layer in catching the sprays. The volume of the sprays which reach the condenser for distillation can be reduced to stably and continuously distill the sulfuric acid liquid.
In a preferred mode of the present invention, the valves of the condensed sulfuric acid removing tank are controlled to be opened and closed in a preset order and at preset time intervals, removing the condensed sulfuric acid from a depressurized state into a normal pressure state. This allows the condensed sulfuric acid to be continuously received into the condensed sulfuric acid receiving tank without stopping the operation of the apparatus for distillation.
When a plurality of baffle plates are arranged in the heating vessel for distillation, the function of the baffle plates is as follows: First, the concentrated sulfuric acid liquid is introduced through the inlet. Next, the concentrated sulfuric acid liquid is stayed by a first baffle plate near to the inlet. Then, the concentrated sulfuric acid liquid is introduced into the space between the first baffle plate and a second baffle plate through the liquid passing hole in the first baffle plate, and the concentrated sulfuric acid liquid is stayed in the space of the first and second baffle plates. After such processes are repeated by the number of the baffle plates, the concentrated sulfuric acid liquid reaches the outlet for the waste sulfuric acid liquid in the heating vessel for distillation. Although the impurity concentration of an unvolatile component in the concentrated sulfuric acid liquid is raised by evaporation due to heating in the heating vessel for distillation, reverse flow mixture of the concentrated sulfuric acid liquid through the liquid passing hole in the baffle plates can be minimized in comparison with the absence of the baffle plates. The impurity concentration of the concentrated sulfuric acid liquid between the first and second baffle plates becomes slightly higher than that at the inlet. After such an increase in the impurity concentration is repeated by the number of the baffle plates, the concentrated sulfuric acid liquid reach the outlet for the waste sulfuric acid liquid in the heating vessel for distillation, through which the waste sulfuric acid liquid with the impurity concentration raised can be stably and continuously removed. This allows the waste sulfuric acid liquid to be continuously supplied to the waste sulfuric acid receiving tank.
In a preferred mode of the present invention, the valves in the waste sulfuric acid removing tank are controlled so as to be opened and closed in a preset order and at preset time intervals, removing the waste sulfuric acid liquid from a depressurized state into a normal pressure state. This allows the waste sulfuric acid liquid to be continuously received in the waste sulfuric acid receiving tank without stopping the operation of the apparatus for distillation.
In a preferred mode of the present invention, the vacuum device works to keep a depressurized state in at least the apparatus for concentration, the apparatus for distillation and the connecting pipe so as to lower the boiling point of the sulfuric acid liquid, reducing the operating temperature. Gaskets which are made of material unlikely to accept impurities can be used in the connecting portions between the apparatus for concentration and the connecting pipe and between the apparatus for distillation and the connecting pipe so as to restrain the liquid from leaking and the respective portions from being damaged, establishing stable operation. If at least the condenser for distillation, the condensed sulfuric acid receiving tank, the condensed sulfuric acid removing tank and the connecting pipes therefor in the apparatus for distillation are made of silica glass, impurities can be restrained from eluting in the reprocessed sulfuric acid liquid, obtaining the reprocessed sulfuric acid liquid in a stable and continuous way.
According to another aspect of the present invention, there is provided a heater supporting structure for a heating device, comprising a heating vessel made of glass; and a heating device inserted in the heating vessel; wherein the heating device is supported at two locations of a heater inserting portion of the heating vessel and another location thereof so as to avoid breakage.
In a preferred mode of the present invention, the heater supporting structure may be constructed so that a leading edge of the heater opposite to the heater insertion portion is inserted in and supported by a bulged portion of the heating vessel. The heater supporting structure may be constructed so that the bulged portion has a cushiony sheath put therein and the leading edge is inserted into the sheath.
In a preferred mode of the present invention, the heater supporting structure may be constructed so that the cushiony sheath is made of PTFE or other fluorocarbon resin.
According to the present invention, the heater inserted into the heating vessel made of glass is supported by two portions of the heater inserting portion and the leading portion, and the stress concentration to the heater supporting portions which is caused by vibration of the liquid in the heating vessel or vibration of the heating vessel per se can be remarkably reduced in comparison with cantilevered supporting. This allows a long heater to be inserted, and breakage due to vibration of the liquid in the heating vessel or vibration of the heating vessel per se can be minimized, permitting intense heating to be carried out.
In the specification, the words xe2x80x9cdevicexe2x80x9d and xe2x80x9capparatusxe2x80x9d have the same meaning as a word xe2x80x9cunitxe2x80x9d, and the word xe2x80x9cdetectorxe2x80x9d has the same meaning as a word xe2x80x9csensorxe2x80x9d.